The synthesis of conjugated oligomers with precisely controlled, well-defined conjugation length has been the subject of growing interest in the conjugated polymer community. A series of oligomers of precisely controlled structure can be used as a model for the investigation of processes governing the physical and photophysical properties of the corresponding larger, polydisperse polymeric materials. Furthermore, monodisperse conjugated oligomers contain minimal structural defects compared to the polymers, and thus allow for a greater control of the material's electronic properties.1 The synthesis of well-defined oligomers typically requires multi-step approaches utilizing many iterations of protection/deprotection chemistry and purification at each step, making such synthesis very low yielding. The development of a one-pot synthetic method towards oligomers with well-defined conjugation length is therefore highly desirable.
Among conjugated polymers (CPs), poly-p-(phenyleneethynylene)s (PPEs) are a class of bright, fluorescent materials with excellent physical and photophysical properties and emerging applications in solar cell electronics, fluorescence analyte sensing and targeted cellular delivery of therapeutics.2 

The synthesis of conventional PPEs utilizes the palladium-mediated Sonogashira coupling reaction between aryl halides and terminal alkynes (AABB-type polymerization). The polymerization under these conditions proceeds in a stepwise manner, requires a high degree of stoichiometric balance, and results in an alternating A-B-type polymer (Scheme 1) with a relatively large polydispersity index.3 
Several approaches to the synthesis of oligo-PPEs have explored different features of the Sonogashira reaction. These include the intentional breaking of stoichiometric balance, differences in reactivity between different aryl halides,4 polymer end-group activation,5 and catalyst transfer polycondensation.6 More precise control can be achieved by step-by-step or convergent synthesis utilizing a series of protection, coupling and deprotection steps.7 All of the above approaches require multiple purification steps, and are thus time-consuming, low-yielding and costly.
PPE polymers can be tailored to a specific application through the modulation of their physical, biological and optical properties by structural modifications of the rigid conjugated backbone and the pendant side-chains. More specifically, controlled introduction of flexibility into the CP backbone with the retention of the optical properties of the fully conjugated PPE polymer is an attractive option for improving the material's solubility, modulating its aggregation properties, or including a biodegradable component for cellular applications.8 Similarly, increased flexible content will translate to the formation of segments of shorter conjugation length, and the precise control of the amount of flexibility will therefore provide a means to control the length of conjugated segments within a polymer chain.
A semi-flexible p-phenylenebutadiynylene (PPB) CP containing a small amount (˜10%) of flexible, non-conjugated units along the polymer backbone has been synthesized.9 The synthesis took advantage of the competing Sonogashira and Glaser-type chemistry under modified Sonogashira coupling conditions, leading to the predominant formation of PPB through the Glaser homocoupling of acetylene monomers with an occasional Sonogashira-type incorporation of a deactivated flexible aryl halide into the backbone. This backbone structure modification influences polymer aggregation behavior and complexation with polyanions,10 and has a dramatic impact on the cellular uptake mechanism and subcellular localization of conjugated polymer nanoparticles (CPNs).11 While these findings warrant a more systematic investigation of the influence of the amount of flexibility on the polymer physical properties, it has not been possible to control the amount of flexible component due to the nature of the catalytic system (Glaser coupling).